1. Field of the Invention
The present invention relates to the production of porous carbon structures and particularly to the manufacture of resin bonded fibrillar carbon paper. More specifically, this invention is directed to fibrillar carbon fuel cell electrode substrates. Accordingly, the general objects of the present invention are to provide novel and improved methods and articles of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to the production of carbon or graphite "paper" . Such carbon "paper" has been employed as electrode support plates and could be substituted for the present porous sintered nickle electrolyte reservoir plates in fuel cells. For a fuel cell application, carbon "paper" must, in addition to being cost effective, meet rather stringent chemical, physical and electrical property requirements. Thus, by way of example, material for use in a fuel cell as an electrode support plate must be characterized by a high degree of resistance to oxidation, availability in a thickness range of 15 to 30 mils while retaining sufficient strength to permit handling and have a porosity in the range of 70 to 90%. This high degree of porosity must be obtained, in the case of a fuel cell electrode support plate, with pore size in the range of 15 to 30.mu.. Additionally, the material must be characterized by a lateral resistivity in the range of 0.01 to 0.09 ohm-cm with the preferred resistivity range being 0.01 to 0.06 ohm-cm.
As an example of the utilization of porous carbon "paper" having the above outlined characteristics, in acid electrolyte fuel cells such "paper" presently functions as support plates to hold the active catalyst of the fuel cell. Carbon "paper" may also be employed as fuel cell electrolyte reservoir plates. An electrolyte reservoir plate in a fuel cell will contain excess KOH solution within its pores, the pores in the case of an electrolyte reservoir plate having an average size in the range of 3-8.mu., and will serve to assure that the maximum amount of the electrolyte communicates with the cell. The electrolyte reservoir plate is also the media through which the product water produced in the fuel cell is removed. In addition to being lightweight, having high pore volume and KOH compatibility, the electrolyte reservoir plate material must be easily wet and contain a narrow pore size distribution to facilitate delivery of electrolyte from its structure to the cell matrix on demand.
The conventional prior art manner of forming porous carbon structures suitable for use as fuel cell catalyst support plates consists of forming chopped carbon fibers into paper-like structures and subsequently bonding the fibers together by chemical vapor deposition of a carbon binder phase. Alternatively, porous carbon "paper" may be fabricated by bonding chopped carbon fibers together with a polymer resin which is subsequently pyrolyzed to form a carbon binder. These prior art techniques are inherently slow and costly. A primary contributing factor to the high cost of the prior art techniques resides in the use of expensive graphite or carbon fibers as the starting material. The use of graphite or carbon fibers as the starting material dictates that the production of the porous carbon "paper" included two graphitization steps; i.e., graphitization to produce the starting fiber and then performance of a further pyrolyzation step to convert the binder material to carbon.